It is well known in the art that combining a conventional polymer with an electrically conductive filler can create an electrically conductive composition which exhibits a positive temperature coefficient of resistivity ("PTC"). By way of example, U.S. Pat. No. 4,966,729 to Carmone et al. teaches a conductive PTC polymer which can be an epoxy resin, polyimide, unsaturated polyester, silicone, polyurethane, or phenolic resin doped with fiber shaped conductive materials. The fibers can be carbon fibers, carbon fibers coated with a metal or an alloy, graphite fibers, graphite fibers coated with a metal or an alloy, graphite intercalation compound fibers, metal fibers, ceramic fibers, or ceramic fibers coated with a metal or an alloy. The material is characterized by the fact that the plastic material of the matrix is a thermosetting resin rather than a thermoplastic polymer. The conductive particles preferably have a large size (&gt;1 micron) and are in fibrous form. U.S. Pat. No. 4,658,121 to Horsma et al. describes self-regulating PTC compositions with reduced thermal runaway problems, comprising a cross-linked elastomer, a thermoplastic polymer, and carbon black. The elastomer component may be polyurethane. The carbon filler is identified as Vulcan XC-72, a high surface area species. U.S. Pat. No. 4,545,926 to Fouts, Jr. et al. reveals conductive polymer compositions comprising a polymeric material having dispersed therein conductive particles composed of a highly conductive material and a particulate filler. Fouts teaches the use of carbon black with an average particle size between 0.01 and 0.07 microns.
The PTC property of the composition means that as the temperature of the composition rises, so does the internal resistance thereof. For many of these substances, the flow of electric current therethrough causes the temperature of the material to rise through Joules heating, and therefore the resistance. As the temperature rises, the polymer matrix expands, causing the conductive moieties (usually carbon black) to lose contact with one another. The electrical resistance thus rises, eventually creating conditions similar to an open circuit. The resulting rise in resistance is greater than would be seen in a conventional resistive heating medium.
These characteristics make the composition suitable for many applications including heaters, sensors, and switches. Essentially, when a voltage is applied, the composition emanates internally generated heat, which simultaneously causes the resistance therein to rise. As the resistance rises, the current flowing through the composition is reduced. Eventually, the composition reaches a temperature at which the current is almost completely cutoff, preventing the composition from getting any hotter than the temperature at the current cutoff level.
These compositions that are most suitable for heaters have a certain critical temperature at which point the thermal coefficient of resistivity becomes very large. This creates a turnoff effect for the heater at the critical temperature. The more distinct the change in the thermal coefficient of resistivity, the sharper the turnoff effect for the heater. Despite many advances in the art, the change in the thermal coefficient of resistivity of existing compositions is still far short of ideal. The prior art heaters do not have a sharp turnoff effect. Also the prior art carbon filled polymer matrix heaters exhibit fluctuations in temperature even after the turnoff point is reached.
It is thus an object of the present invention to provide an electrically conductive polymer composition for a heater that exhibits a positive temperature coefficient of resistivity.
It is a further object of the present invention to provide such a composition that has a narrow range of temperatures in which the composition changes from conductive to resistive.
It is yet a further object of the present invention to provide such a composition that exhibits negligible fluctuations in temperature once a critical temperature is reached.
Other objects of the invention will become apparent from the specification described herein below.